Incredibly, optical flickering in old telly limited to one line (9)

I believe the answer is:

extremely

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'incredibly optical flickering in' is the definition.
Both the definition and answer are adverbs. Perhaps they are linked in a way I don't understand?

'old telly limited to one line' is the wordplay.
'old telly' becomes 'extreme' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'limited' means to remove the first letter.
'to one line' becomes 'ply' (ply is a kind of line).
'ply' with its first letter taken off is 'ly'.
'extreme'+'ly'='EXTREMELY'

Can you help me to learn more?

(Other definitions for extremely that I've seen before include "To a high degree or extent" , "remarkably" .)

I've seen this clue in The Independent.

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